This invention relates to secondary (rechargeable) lithium batteries which utilize vanadium oxide intercalation compounds as the positive battery electrode. In particular, the invention provides means for improving the performance and energy density of such batteries through the use of a new class of intercalation electrodes comprising copper vanadium bronzes.
Rapid growth in the use of electrical products, ranging from electronics instrumentation, such as sophisticated telecommunication equipment and computers, to powered vehicles, has generated a wide-spread requirement for self-contained power sources and means for renewable energy storage. Therefore, there is a critical need for low-cost, rechargeable energy storage devices (batteries) that have high energy densities and can deliver power reliably at a constant voltage over many recharge cycles.
Due to their favorable weight advantage and significant energy densities, the lithium intercalation batteries have been extensively studied, and substantial investigation has been carried out with such devices comprising vanadium oxide cathodes. Promising results have been obtained with the V.sub.2 O.sub.5 class of cathode compounds and much recent work has been conducted to improve the capacity and other performance characteristics of these materials. Of particular interest are attempts to improve the capacity of the positive electrode vanadium oxides to intercalate lithium ions, the primary determinative of energy density in these rechargeable batteries.
Several phases of the Li-based vanadium oxide system derived by room temperature insertion of lithium ions into the various V.sub.2 O.sub.5 structures have been characterized by X-ray diffraction examination. The .alpha.-, .epsilon.-, and .delta.-Li.sub.x V.sub.2 O.sub.5 phases have been shown to have Li levels of 0&lt;x&lt;0.1, 0.35&lt;x&lt;0.5, and 0.9&lt;x&lt;1. Some investigators, (C. Delmas et al., Journal of Power Sources, 34 (1991) 113-118) have achieved higher Li intercalation, i.e., in the range of 2.ltoreq.x.ltoreq.3, by means of abnormally excessive discharge of V.sub.2 O.sub.5 -based cells to derive an .omega.-Li.sub.x V.sub.2 O.sub.5 phase composition. The achievement of such desirable V.sub.2 O.sub.5 phases have, for the most part, required electrochemical processing of fabricated Li/V.sub.2 O.sub.5 cells, however. As a result, there has been no practical means for obtaining economically useful amounts of these improved materials. The more recent development of a .gamma.' phase of V.sub.2 O.sub. 5 by chemical oxidation of .gamma.-Li.sub.x V.sub.2 O.sub.5 (J. M. Cocciatelli et al., Journal of Solid State Chemistry, 93 (1991) 497-502, has shown promise as a means for achieving an effective level of Li intercalation which has been targeted as at least one Li per vanadium in the electrode formula.